1. Field of the Invention
The present invention relates to a water-soluble organic binder used in molding ceramics. The present invention also relates to a method for producing ceramics which are sintered products.
2. Brief Description of the Prior Art
As one molding method of ceramic products which are produced by sintering raw sheets or green sheets comprising sinterable fine inorganic oxide powder such as alumina, silica, magnesia and beryllia, there is a sheet molding method which comprises mixing a fine sinterable powder and a solvent to prepare a slurry, applying the latter in the form of a sheet by use of a doctor blade or the like, and thermally drying to obtain a raw sheet or green sheet. The thus obtained sheet is afterward, for example, punched to be formed into a desired shape, followed by sintering. The organic binder contained in the green sheet is extinguished by firing in the course of sintering. In this case, therefore, the organic binder is employed so as to retain the given shape until the sintering finishes, in particular during the punching.
For example, in the conventional sheet molding method, an organic binder such as a butyral resin or a vinyl acetate resin is dissolved in an organic solvent such as methyl ethyl ketone, butyl acetate, ethyl acetate, toluene or an alcohol, and the resultant solution is then mixed with a sinterable fine powder over a long period of time in order to form a slurry. After deaeration, the slurry is applied onto a carrier film such as polyester film by means of a blade in the form of a sheet having a predetermined thickness, and it is heated for drying or naturally dried to prepare a sheet (a raw sheet or green sheet).
However, this method in which the organic solvent is used involves problems of (1) harm of the organic solvent to a human health, (2) danger such as explosion due to employment of the flammable organic solvent, and (3) being uneconomical owing to utilization of the expensive organic solvent.
Hence, with the intention of overcoming the above-mentioned drawbacks, some additional methods have been suggested in which there are employed water-soluble organic binders such as polyvinyl alcohols and polyvinyl acetates, and aqueous emulsions, as binders, such as emulsions of polyacrylates and copolymers of ethylenes and organic acids.
For example, in U.S. Pat. No. 4,010,133, an acrylate copolymer of 50 to 7% alkyl acrylate ester, 20 to 50% alkyl methacrylate ester, and 0.2 to 2% acrylic acid and methacrylic acid is used as an organic polymeric binder to prepare an aqueous slurry of a barium titanate powder, and from the slurry, the green sheet is prepared.
These suggested methods an solve the abovementioned problems (1), (2) and (3) because water is used as the solvent, but they instead have brought about other problems. For example, in longtime mixing of the sinterable fine powder, the latter will tend to coagulate in the slurry due to the presence of water, and the viscosity of the slurry will thus deviate from Newtonian flow, with the result that fluidity of the slurry and dispersion of the powder will deteriorate and it will be difficult to prepare the sheet having a high density and smooth surfaces. Further, in such methods above, a sheet having a good flexibility and a great mechanical strength is scarcely manufactured.
That is to say, essential requirements for the ceramic sheet molding, in addition to consideration of the above-mentioned factors (1), (2) and (3), are:
(4) that the dried ceramic sheet has smooth surfaces and high density;
(5) that the molded sheet has a proper flexibility and mechanical strength for the sake of subsequent press lamination and tape winding; and
(6) that the molded sheet can easily revert to its original slurry.
However, in the case of the emulsion type organic binder, it is hard to recycle to the slurry. Accordingly, the manner of using such a binder type can be concluded to be unsuitable for sheet molding.
On the contrary, the molding method of using a water soluble organic binder and water as a solvent is preferred because reproduction of the slurry is possible.
However, with regard to a polymer containing a carboxylic acid such as acrylic acid, which is often employed as the water-soluble organic binder, its carboxylic acid group (carboxyl group) is neutralized with an alkali such as sodium hydroxide or potassium hydroxide, or ammonia to impart the water-soluble property. It is here to be noted that such a neutralization will disadvantageously prevent the production of the flexible sheet.
Becher et al., in U.S. Pat. No. 4,339,365, discloses a coating composition containing an acrylate copolymer of acrylic acid. The use of the copolymer as a binder, however, is not disclosed therein.
In ceramic sheet molding, it is also important for the organic binder to be easily decomposed during sintering. The organic binder of the present invention is easily decomposed with no carbon deposit when fired.